In order to attain a reduction in the weight, an improvement in the fatigue strength, etc., there is an ever-increasing demand for an increase in the strength of an ultra fine steel wire. A very fine steel wire used for the reinforcement of tires of automobiles, various belts for industries, etc., has hitherto been produced by subjecting a hot-rolled wire material of a high carbon steel to a repeated intermediate wire drawing and a patenting treatment to bring the wire into a desired wire diameter and then subjecting the wire to a final patenting treatment, plating the treated wire for improving the wire drawability and the adhesion to rubber, and subjecting the plated wire to wet drawing to a predetermined wire diameter. For example, the steel tire cord is produced by finally twisting the element wire produced by the above-described method by a twisting machine, such as a double twister.
In the above-described manufacturing process, in order to attain an increase in the strength of an ultra fine steel wire, it is necessary to increase the strength of an element wire after the final patenting treatment or to increase the final wire drawing strain, but an increase in the strength of the element wire after the final patenting treatment or the wire drawing strain frequently gives rise to a breaking of a wire in the step of twisting after the wire drawing, which remarkably lowers the productivity. For this reason, for example, in the case of a steel wire having a wire diameter of 0.3 mm.phi. wherein use is made of SWRS82A, the limitation of the tensile strength sufficient to withstand twisting is 340 kgf/mm.sup.2, and it is difficult to produce an ultra fine steel wire having a higher strength on a commercial scale.
On the other hand, in order to improve the workability in stranding of a high carbon steel wire having an increased tensile strength, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 60-204865 and 63-24046 and Japanese Examined Patent Publication (Kokoku) No. 3-23674 propose a high carbon wire material for an ultra fine wire less liable to breaking in the step of twisting, through the regulation of chemical ingredients such as C, Si, Mn and Cr. As apparent also from working examples, the tensile strength of the steel wire is 350 to 360 kgf/mm.sup.2 at highest, which limits an increase in the strength of an ultra fine steel wire. Further, in order to improve the fatigue properties of the ultra fine steel wire, for example, Japanese Unexamined Patent Publication (Kokai) No. 2-179333 proposes a process for continuously producing an ultra fine wire having a high fatigue resistance, through a continuous projection of fine hard particles onto the surface of an ultra fine wire having a diameter of 0.5 mm or less to improve the residual tensile stress of the surface layer of the extra thin wire into a residual compression stress. Nevertheless, to convert a residual tensile stress in the surface layer of a high-strength of 400 kgf/mm.sup.2 or more, ultra fine steel wire to a residual compression stress, it is necessary to conduct a high degree of shot peening, so that problems arise such as an increase in the surface roughness and peeling of a brass plating in the surface layer having a reduced thickness due to wire drawing.